High-pressure gas discharge lamps need to be started by a high voltage which is provided by a starting device. After starting, the lamp is heated during a start-up phase from a starting temperature to an operating temperature. The voltage applied to a lamp after starting is referred to as the running voltage and is, within wide limits, not substantially dependent on the lamp current. The running voltage increases during the start-up phase from a starting running voltage to an operational running voltage. The start-up phase is followed by an operating phase in properly functioning gas discharge lamps.
In lamp technology, a distinction is drawn between high-pressure and low-pressure gas discharge lamps. With high-pressure gas discharge lamps, it is essential for operation that, during the start-up phase, the pressure in the lamp vessel increases from an initial pressure to an operating pressure. This is one reason why the invention described below can be used in a particularly advantageous manner in the case of high-pressure gas discharge lamps. However, it is also possible for it to be used in the case of low-pressure gas discharge lamps.
During the operating phase, it is conventional for the operating device to regulate the power of the lamp such that it is at a desired power. Since the running voltage is low during the start-up phase, a high lamp current is required in order to set the desired power during the start-up phase when there is power regulation alone. This current may be a multiple higher than the lamp current during the operating phase. This would lead to destruction of the electrodes of the lamp. Therefore, in the prior art, the current provided to the lamp by the operating device during the start-up phase is limited to a constant start-up current. At least during a first section of the start-up phase, the lamp is thus fed the constant start-up current. During the course of the start-up phase, the running voltage increases. If the running voltage reaches a value which, together with the constant current, produces the desired power, the power regulation begins to operate. In the event of a further increase in the running voltage, the lamp current is reduced to such an extent by the power regulation that the desired power is set. The start-up phase is concluded if the running voltage has reached the value of the operational running voltage. The operational running voltage has manufacturing tolerances and also changes during the life of a lamp. The operational running voltage is therefore defined by the running voltage which remains essentially constant at the desired power. In order to eliminate fluctuations, the running voltage is usually measured as a mean value over time. An operating lamp current correlates with the operational running voltage and, together with the operational running voltage, produces the desired power.
The following needs to be taken into account for the value of the start-up current: during the start-up phase, so much power needs to be injected into the lamp that the pressure in the lamp and thus the running voltage continuously increase until the operational running voltage has been reached. Otherwise, it may come about that the lamp remains in a stable state during the start-up phase and the desired power is not reached. In order to reliably rule out this situation, a start-up current is selected in the prior art which is markedly above the operating lamp current. This is illustrated in the specification U.S. Pat. No. 5,083,065 (Sakata). In this specification, an operating device is described which has no power regulation but the lamp current is merely set via the operating frequency. A control unit detects the increase in the running voltage throughout the start-up phase and increases the operating frequency if the increase in the running voltage is too great. The value of the lamp current is thus limited indirectly.
One aspect when selecting the start-up current is also the desire for a start-up phase which is as short as possible in order to achieve a desired luminous flux in as short a time as possible. This is achieved by a high start-up current. A high start-up current represents a severe load on the electrodes, however, which leads to damage to the electrodes and thus reduces the life of a lamp. The electrodes are damaged either by overheating, which leads to fusing and erosion, or by so-called sputtering, which is caused by ions hitting an electrode at high speed.
With operating devices according to the prior art, the start-up operation is disruptively long for many applications.